When ''Children Underground'' was filmed three years ago, Aurolac, which is ruinous to the lungs and produces a chronic, hacking cough, was the drug of choice among the city's homeless youth. Since then, the movie's epilogue says, heroin has taken over.

The members of this loosely knit pack eke out an existence by begging and doing occasional odd jobs for storekeepers. They sleep in a subway station on slabs of cardboard and bathe by pouring water from plastic soda bottles over their hands and faces. Most are lice-infested. In the hardheaded view of the few agencies devoted to helping the city's 30,000 homeless children, reclaiming a child who has lived on the streets for very long is all but impossible, especially when drugs are involved.

The misery ''Children Underground'' records is a bitter legacy of the despotic Ceausescu regime, which outlawed contraception and abortion as part of a program to increase the country's work force. As a result, thousands of unwanted children were born to parents living in such poverty that they were unable to care for their offspring. Many were deposited in state-run orphanages. Since 1989, Romania's failure to adjust to a free-market economy has left the country one of the poorest in Eastern Europe.

''Children Underground'' goes out of its way not to sentimentalize its subjects. Its visual rhythms are harsh and jagged. It doesn't try to impose a tight narrative structure and has no music. There are no caring mommies and daddies waiting in the wings to make things better.

All five of the children under examination appear seriously damaged. The most colorful is the group's swaggering self-appointed leader and stern den mother, Cristina, who was raised in an orphanage and has been living on the street since she was 11. To protect herself from sexual assault, Cristina, who is 16, has shorn her hair and tries to pass as a boy.

The child who seems most salvageable is 12-year-old Mihai, a smart, spunky runaway who doesn't sniff paint and who dreams of going to school and having a job. But when Mihai, who left home at 8 or 9 to escape his father's beatings, is offered the chance to reunite with his family, he balks in terror. The film visits his glum, impoverished family (the father is away), and you sense deep trouble.

The youngest members of the pack, also runaways, are a sister and brother, 10-year-old Ana and her younger brother, Marian, who is 8. The pair are pathetic, frightened creatures who alternately cling to each other and fight. When the film visits their family, the gaunt, hard-boiled stepfather claims to care about them but describes Ana as psychopathic.

''Children Underground,'' which opens today at Film Forum, is more uncomfortable to watch than many documentaries because the filmmakers, who were obviously in a position to change the lives of their subjects, stand back and do nothing beyond aiming their cameras. That passivity in the face of such misery inevitably becomes a theme of the movie. It's one thing for a news crew to pay a detached visit to a scene of social devastation, then depart, and another for a film crew to spend a year observing a group of lost children without trying to save them.

Ms. Belzberg, to her credit, directly addresses that question in the movie's production notes. ''The decision not to intervene was one of the biggest personal challenges I faced in making this film,'' she writes. ''I realized early on that if we were to intervene, it would make a difference for the children only in the very short term, and meanwhile, because the crises were so relentless, the film would not get made.''

This sad declaration of ultimate impotence is supported by everything seen on the screen, and it makes ''Children Underground'' a singularly depressing film. In the face of such unrelieved, grinding poverty, hope fades.

CHILDREN UNDERGROUND

Produced and directed by Edet Belzberg; in Romanian, with English subtitles; director of photography, Wolfgang Held; edited by Jonathan Oppenheim; released by HBO/Cinemax Documentary Theatrical Presentations, in association with Red Horse Inc., and Childhope International. At Film Forum, 209 West Houston Street, South Village. Running time: 108 minutes. This film is not rated.